how to figure telephoto focal length; with samples

[fish4570]

i'm still looking for a cost-efficient tele for photographing our resident hawks when their fledglings hatch. i've good good 7x35 nikon binoculars for just watching.
this morning, i took my praktica with vivitar 135, on a tripod, just to see what the aerie looks like at that focal length from several viewing spots, measured in meters from the base of the tree.
the closts viewing spot is 24 meters. the aerie is about the size of the fresnel circle in the praktica's frame, looking through the 135. i even snapped a shot from each of 5 spots that i will put up later this week. the farthest spot is about 44 meters.
i'd like the aerie to be at least three times the relative size of the aerie in the viewfinder with the 135 as it is now from 24 meters. the aerie is maybe 20-22 meters up the tree.
some of you guys who know geometry and lens geometry help me here? do i need a 300? 500?

[Boomer Depp]

http://www.cambridgeincolour.com/tutorials/camera-lenses.htm

If you were photographing a .5 foot bird 50 feet away and wanted to fill the frame,by the calculator in the above link,it appears you need a lens with focal length f = 1817.9mm (Yikes!)...for 75 feet a 2726.9mm(excuse me while I faint)

from wikipedia....http://en.wikipedia.org/wiki/Angle_of_view

Let s = the sensor size in its longest dimension
Let h = the longest subject dimension (equals subject height in portrait mode)
Let f = the focal length
Let S1 = the distance from the lens to the subject
Let S2 = the distance from the lens to the sensor
Let alpha = the angle of view


EQN1: alpha/2 = arctan(s / (2*S2)) = arctan(h / (2*S1))

EQN2: 1/f = 1/S1 + 1/S2


So, for example, suppose I wanted to photograph a 0.5ft-tall bird from 50ft away with a camera with a 1.6x lens multiplier.

On a 35mm camera the horizontal sensor length is 36mm, so on a camera with a 1.6x multiplier, the sensor is 36/1.6 = 22.5mm long = s.

S1 = 50ft.

The EQN1 calculation for 50 feet for a 1.6 crop sensor body, S2 = s*S1 / h = 22.5mm * 50ft / 0.5ft = 2250 mm (Unfathomable Yikes!)....75 feet a 3375mm


By the EQN2 calculation at 50 feet for a full frame body....EQN2: 1/f = 1/S1 + 1/S2....it appears you need a lens with focal length f = 1961mm (Double Yikes!)...75 feet a 2946mm


This is why bird photographers prefer a solid 500mm or more telephoto lens coupled with a solid full frame DSLR or a 35mm film body,so they can crop the images.

So without going the expensive telephoto route or the inexpensive preset long focal length lens....at the least a good quality 500mm Mirror lens is needed.

[fish4570]

well, it's an aerie that i'd like to fill at least half the frame. the aerie is at least two feet wide - better than half a meter.
the hawk parents are better than a foot tall ...

[visualopsins]

Bargain price, best quality non-mirror lens under US$100:

Spiratone 6.3/400 Sharpshooter T-mount lens with Spiratone T-mount 2X TC. Effectve f=800mm. Lightweight. Spiratone sold a bazillion of these, because they offer good quality and are inexpensive. Imho, better than mirror lens, and no doughnut-shaped bokeh.

T-mount adapters are available for every 35mm SLR...

photos of lens: http://www.orionsword.net/Astronomy/Equipment/Pages/Sharpshooter1.html

astrophotos from Sharpshooter 400: http://www.orionsword.net/Astronomy/Equipment/Pages/Sharpshooter.html

[Boomer Depp]

A 500mm or better(and I've known bird photographers who've stacked two TCs on a 500mm)...or a 2x TC coupled to a solid 300 or 400mm lens.

[William]

OT but thank you Boomer Depp for your very informative post earlier. I am planning on doing some extreme long distance photography experiments when I have the equipment such as shooting major landmarks from several miles away at different angles and distances and taking a scientific approach. Your link and mathematical explanations will help with this.

Fish4570, I eagerly anticipate your hawk pictures, they sound like a fair challenge!

[fish4570]

i put a bid on a spiratone sharpshooter.

this is one of the fledglings from last year after its first flight. it stayed on the rail at a neighbor's cabin for about an hour. i shot it with either a pentacon 58/1.8, or vivitar 135/2.8. can't remember.







a newspaper photographer friend of mine came out and shot the fledglings being fed by the parents. they got good play ...

[fish4570]

bid is holding up with less than 2 hours to go ...

[Boomer Depp]

I'm watching it and another...hope you have the high bid....good luck!

BTW I'm not bidding...just watching.

[fish4570]

thanks!

[Boomer Depp]

All the best to you! BTW...Looks like an excellent copy of the lens!

[fish4570]

noticed with binoculars a few minutes ago the presence of breast feathers on the aerie. the female may already have laid her eggs. the male and female take turns sitting the eggs, while the other hunts.

http://www.allaboutbirds.org/guide/Red-shouldered_Hawk/lifehistory

[fish4570]

got it!

[Boomer Depp]

Congrates!Looking forward to some pics!

[fish4570]

ordered the adapter ...